Image density control

ABSTRACT

A photoelectrophoretic imaging apparatus comprising two additional electrodes for increasing the density and reducing background level of an image reproduced at high speeds.

United StatesPatent 1191 [11] 3,784,294 Wells Jan. 8, 1974 IMAGE DENSITYCONTROL [561 References Cited [75] Inventor: John B. Wells, Rochester,NY. UNITED STATES PATENTS 3,598,579 8/1971 Robinson 355/3 [73] Assgneexemx Cmpomuon Rochester 3,620,948 11/1971 Walsh 204/181 PE [22] Filed:Oct. 4, 1971 3,595,772 7/1971 Zucker 96/13 [21] Appl' NO; 186537 PrimaryExaminerl-loward S. Williams Related U.S. Application Data AssistantExaminer-W. I. Solomon [62] Division of S81. N0. 863,506, 01:1. 3, 1969,Pat. NO. Attorney-Richard Tomlin i [57] ABSTRACT [52] U.S. Cl 355/3,96/1 PE, A photoelectrophoretic g g apparatus Comprising two additionalelectrodes for increasing the density and reducing background level ofan image reproduced at high speeds.

15 Claims, 1 Drawing Figure IMAGE DENSITY CONTROL This is a division ofapplication Ser. No. 863,506, filed in the U.S. Oct. 3, 1969, now US.Pat. No. 3,645,874.

BACKGROUND OF THE INVENTION uid. This suspension is then placed betweenat least two electrodes subjected to a potential difference and exposedto a light image. Ordinarily, in carrying out the process the imagingsuspension is placed on a transparent electrically conductive support inthe form ofa thin film and exposure is made through the transparentsupport while a second generally cylindrically shaped biased electrodeis rolled across this suspension. The particles are believed to bear aninitial charge once suspended in the liquid carrier which causes them tobe attracted to the transparent base electrode and upon exposure, tochange polarity by exchanging charge with the base electrode so that theexposed particles migrate to the second or imaging electrode therebyforming images on each of the electrodes, by particle subtraction, eachimage being complementary one to the other. The process may be used toproduce both polychromatic and monochromatic images. In the latterinstance a single color photoresponsive particle may be used in thesuspension or a number of differently colored photoresponsive particlesmay be used all of which respond to the light to which the suspension isexposed. An extensive and detailed description of thephotoelectrophoretic imaging techniques as generally referred to may befound in US. Pat. Nos. 3,383,993, 3,384,488, 3,384,565 and 3,384,566,and are hereby incorporated by reference. i

In the case of the polychromatic imaging process the imaging suspensionwill contain a plurality of at least two differently colored finelydivided particles in the carrier liquid each of said particlescomprising an electrically photosensitive pigment whose principal lightabsorption band substantially coincides with its principalphotosensitive response. Thus, the pigment represents both the primaryelectrically photosensitive ingredient and the primary colorant for thespecific particle in suspension. The particles utilized in thepolychromatic system preferably have intense pure colors and are highlyphotosensitive. When the suspension is exposed to a multicolored image,particles will migrate to one electrode in proportion to the intensityof the light which they absorb. Thus, upon exposure, particlesselectively remain on one of the electrodes in image configuration withcomplementary particles migrating to the other of the electrodes in thissystem. For example, when a mixture comprising cyan, magenta and yellowparticles is exposed to an image whereby yellow light impinges theimaging suspension, the cyan and magenta particles will migrate leavingbehind an image made up of the yellow pigment particles. Similarly, whenexposed to a multicolored image different color particles absorb lightof their complementary color in the appropriate image areas and migratethereby leaving a full colored image behind corresponding to theoriginal.

Although the above described imaging systems have been found highlysatisfactory for producing acceptable images one of the more troublesomeproblems encountered is to obtain high quality images with lowbackground while maintaining the density of the resulting imagessufficiently high to produce the necessary image contrast. In bothmonochrome and polychrome duplicating processes in order to reducebackground at the operating speed of the duplicator a film-splittingroller has been introduced into the system which generally provides anadequate control to reduce the background level. However, in order toachieve the results desired at the operating speeds of the duplicator itis necessary to control the amount of. ink flow (photosensitizingparticles) presented for imaging. This requirement in effect limits theconcentration of the electrophoretic pigment particles present 'in theimaging suspension at the imaging zone thus reducing the ultimatedensity of the resulting image.

SUMMARY OF THE INVENTION It is, therefore, an object of this inventionto provide an electrophoretic imaging system which will overcome theabove noted disadvantages.

It is a further object of this invention to provide an electrophoreticimaging process capable of producing high quality, dense images.

It is another object of this invention to provide a novelelectrophoretic imaging process.

Yet still a further object of this invention is to provide a novelelectrophoretic imaging apparatus.

It is still a further object of this invention to provide a highcontrast imaging system.

The foregoing objects and others are accomplished in accordance with thepresent invention, generally speaking by providing an imaging suspensioncomprising colored photoelectrophoretic imaging particles in aninsulating carrier liquid. The imaging suspension of the presentinvention is interpositioned between at least two electrodes one ofwhich is generally substantially transparent, subjected to a potentialdifference and selectively exposed to a. reproducible image by a sourceof activating electromagnetic radiation. The imaging suspension isgenerally coated on the surface of a first transparent electrode in theform of a thin film and the exposure made through the transparentelectrode generally during the period of contact with a second orimaging electrode. Prior to image exposure, the coated layer of imagingsuspension is first subjected to an electric field generated by apotential generally below the corona threshold of the film. The effectof this field is to control the quantity of particles present in thesuspension so as to, in effect, increase the concentration of theparticles in the suspension for a givenfilm thickness and to therebymaximize the density of the resulting image without effecting in adetrimental manner the other desirable operating characteristics of thesystem such as the operating speed. Following exposure to this densitycontrol electrode the film of imaging suspension is subjected to asecond electric field by way of still another electrode at a potentialgenerally above the corona threshold of the film thereby establishingthe necessary corona discharge current which splits or otherwiseseparates the imaging suspension into two layers wherein thephotoelectrophoretic pigment particles are effectively rendered unipolarand are substantially concentrated in a uniform manner on the surface ofthe transparent electrode. Thus, the effect of the second field andresulting current applied is to cause an electrophoretic deposition ofthe imaging particles in the form of a uniform film on the respectiveelectrode thereby creating in essence a two-layered film consisting ofunipolar particles and vehicle, respectively. The photomigratoryparticles present in the suspension next respond to the exposureradiation in the imaging zone to form a visible image pattern at one orboth of the electrodes, the images being complementary in nature. Theimaging suspension employs in tensely colored pigment particles whichserve both as the colorant and as the photosensitive material.Additional photosensitive elements or materials are not required thusproviding a very expedient imaging process. The particles respond tolight in the regions of the spectrum of the principal absorption bandwith, for example, cyan, magenta and yellow particles responding to red,green and blue light, respectively. Thus, ifa specific pigment isimpinged by white light then it can be expected to respond to produce animage.

It has been determined that upon subjecting the imaging suspension ofthe present invention to an electric field of sufficient magnitude priorto the imagewise exposure of the suspension that the concentration ofthe pigment particles in the suspension may be so controlled such thatat high speeds a sufficient amount of the photoconductive pigmentparticles are present in the imaging film layer so as to produce highdensity images. The effect of the field is to increase the density ofthe suspension beyond that which would normally be attained by coatingthe electrode surface in a conventional manner. When used in the courseof the present invention the expression corona threshold potential orvoltage" refers to that voltage at which air ionization occurs in theair gap between the particular liquid film and the respective electrodesurface.

DETAILED DESCRIPTION OF THE INVENTION The invention is furtherillustrated in the accompany ing drawing in which there is seen acontinuous electrophoretic duplicator comprising a transparent injectingelectrode 1, imaging electrode 10, a film-splitting electrode 20 and animage density control electrode 40. The transparent electrode 1, in theinstant illustration, is represented as consisting ofa layer ofoptically trans parent glass 2 overcoated with a thin opticallytransparent layer of tin oxide 3. Tin oxide coated glass of this natureis commercially available under the trade name NESA" glass. A uniformlayer of the imaging suspension 5 of the present invention is coated onthe surface of the transparent electrode by an applicator 6 of anysuitable design or material, such as a urethane sponge coated cylinder,which may rotate in the same'direction as the transparent cylinder, or,as herein represented, in the opposing direction to the transparentcylinder. The function of the ink applicator is to apply the imagingsuspension 5 from ink sump 7 by way of roller 8 to the transparentcylinder. In close proximity to the transparent roller electrode 1 is asecond rotary electrode 10 having a conductive central core 11 which iscovered with a layer 12 of material capable of blocking DC current, suchas polyurethane, which will be referred to as a blocking layer. Althougha blocking layer need not necessarily be used in the system, the use ofsuch a layer is preferred because of the markedly improved results whichit is capable of producing. A delayer may be found in US. Pat. No.3,383,993.

The imaging suspension will consist of a dispersion of specificallycolored, finely divided photosensitive particles in an insulatingcarrier liquid or vehicle. Any suitable differently coloredphotosensitive pigment particles may be used such as disclosed in US.Pat; Nos. 3,384,565 and 3,384,566. When the system is to be used in itspreferred mode in conjunction with monochromatic photoelectrophoreticimaging then the imaging suspension will contain a plurality of pigmentparticles in a carrier liquid the pigment portion of which provides boththe photosen'sitivity and colorant property for the particles. In thecase of a polychrome system the suspension will contain a plurality ofat least two differently colored particles having similar properties tothose used in the monochrome process. If desirable a polychrome imagemay be prepared according to monochrome imaging in registrationutilizing the proper color separation negatives as disclosed in US. Pat.application Ser. No. 812,796, filed Apr. 2, 1969 and now abandonedhaving a common assignee, or the input may be in the form of aKo'dacolor negative. In an alternate embodiment, the suspension may becoated on the imaging electrode as depicted in U.S. Pat. No. 3,427,242,with the appropriate biasing electrodes added, whereby the color imageis produced by a back migration of the image particles to the surface ofthe transparent roller electrode. Although not preferred thelatteralternate embodiment demonstrates the flexibility of the system.The imaging suspension may also contain a sensitizer and/0r binder forthe pigment particles. The percentage of pigment in the carrier is notconsidered critical; however, for reference purposes it is noted thatfrom about 2 to 10 percent pigment by weight has been formed to produceacceptable results.

A receiver sheet 13 is driven between cylinders 1 and 10 as represented,with an ink image selectively deposited on the receiver sheet in theimaging zone. A reverse image pattern is formed on the NESA glasscylinder which is removed at the ink application station. Thus, theapplicator performs both the ink application and residual image removalsteps.

Situated in close proximity to the applicator roll is a third electrodegenerally designated 40 and hereinafter referred to as the densitycontrol roller. Roller 40 consists of a conductive central core 41covered with a layer of a dielectric material 42. Any suitabledielectric material may be used. Typical dielectric materials includeelastomeric materials such as polyurethane elastomer (DisogrinIndustries); silicone rubber RTV (General Electric Co.); Neoprene, atype of elastomer based on polymers of2-chlorobutadienel,3;fluorelastomers such as Dow Cornings fluorosiliconeelastomers and Viton available from duPont; natural and vulcanizedrubbers; polyvinylfluoride plastics such as Tedlar (duPont) and KYNAR(Pennsalt Corp.); polyester plastics such as polyurethane (Witco Co);acrylonitrile polymers such as Hylar (BF. Goodrich); mixtures andcopolymers thereof.

As the film of the imaging suspension 5 passes beneath the densityroller 40 a potential is applied to roller 40 by source 45. The effectof the resulting field established across the suspension is to cause anelectrophoretic separation of the photo-conductive particles from theliquid onto the transparent electrode producing a dense coating forfurther processing without effecting total film thickness. In thismanner the optical density of the ink film applied may be effectivelycontrolled so as to ultimately produce the maximum image density. Thedensity of the ink 'film may be continuously monitored such as by aphotoelectric means consisting of a lamp and photocell (not shown) and asignal fed to a control which regulates the potential on the biaseddensity roller in a manner which maintains the concentration of pigmentin the suspension relatively constant and at the desired level. Thedensity roller may also serve to regulate the film thickness by itsmechanical spacing and pressure so as to maintain the ink film thicknessuniform in a manner compatible with the operating speed of the system.Thus the density roller may serve a twofold purpose-that of increasingand maintaining uniform maximum print density as well as uniform filmthicknesses. In addition, a metering device as represented by means 26may be utilized in conjunction with the density roller so as to assurethe proper film thickness for corresponding operating speeds.

The polarity of the potential applied to the density roller may beadjusted depending upon the polarity of the particles dispersed in theimaging suspension. The magnitude of the voltage will generally be lessthan the corona threshold for the air gap between the liquid film androller. Sufficient voltage is applied so as to electrophoreticallydeposit the particles in the suspension on the surface of thetransparent electrode 1 thereby increasing the concentration of pigmentin the suspension to a degree substantially greater than that whichordinarily would be present in the absence of the applied field. The endeffect on the density of the suspension will depend upon the magnitudeof the potential applied to the roller and the controlled filmthickness. Voltages effectively applied in the course of the presentinvention at film thicknesses of up to about microns were generally lessthan 2500 volts. As stated above, the polarity of the potential appliedto the density roller will generally be determined by the particularpigment particles dispersed in the carrier liquid and need notnecessarily be the same polarity of the potential applied to thefilm-splitting roller further discussed below. Due to the presence ofthe density control roller in the system it is now possible to produceimages at a rate much more rapidly than heretonow thought possible.However, imaging speeds as high as 155 ips have been achieved. Highquality images have been obtained at speeds of up to 40 to 50 ips.

Located in close proximity to the area of contact of the transparent andimaging electrodes is still a fourth electrode generally designatedconsisting of a conductive central core 21 covered with a layer ofdielectric material 22. The dielectric materials utilized here aresimilar to those referred to above with respect to electrode 40. Thiselectrode is generally referred to as the film-splitting electrode. Asthe film of imaging suspension 5 coated on the surface of thetransparent electrode 1 passes beneath the film-splitting electrode 20,a DC. potential is applied to the latter electrode by potential source25. The effect of the resulting filed and established corona currentacross the air gap and the surfaces as herein represented, will besubstantially free of pigment particles thereby minimizing thepossibility of contaminating the image support, surface. A means 26 formetering the ink flow passing between the filmsplitting electrode andthe transparent electrode may be included in the system to provide abackup system for the metering effect of the density electode. Controlof the ink film thickness is necessary to eliminate ink flooding whichtends to suppress corona and thereby nullify the effect of the coronacurrent upon the imaging suspension generated at the film-splittingelectrode.

The potential applied to the film-splitting electrode is generallymaintained at a value above the corona threshold potential 'for the airgap between the liquid film 5 and roller 20. The primary concern is thatsufficient D.C. corona current be generated to cause the particles inthe suspension to become unipolar and to establish the two-layered film.Voltages effectively applied to the course of the present invention atfilm thicknesses of about I to 2 microns are generally greater than 2500volts. At inkfilm thicknesses greater than Zmicrons corona threshold isgenerally found to be somewhat greater than 3500 volts. For maximumassurance that the desired effect is realized preferred voltages are inthe range of from about 5000 to 8000 volts. The polarity of thepotential applied to the filmsplitting roller is generally maintained atthe same as that applied to the imaging electrode 10.

The layered suspension enters the imaging zone beween the transparentand imaging electrodes with the vehicle being the outermost layer. Animage is projected into the nip of the rollers by way of a first surfacemirror designated 39. A filed is established across the imaging zonewith the potential being supplied by power source 35. Through the entireoperation the NESA glass transparent roller electrode is connected toground. If desirable the ground connection may be eliminated and a biasapplied to the NESA electrode. A receiver sheet 13 represented in theform of a paper web is fed from supply roll 36 and passes between theglass injecting electrode and the imaging electrode and is rewound ontake up roller 37. Fixing of the image developed on the surface of thecopy web 13 may be acceleraed by the presence of heating unit 38 whichassists in vaporizing the carrier component remaining in combinationwith the colored pigment particles.

Although the film-splitting roller may be positioned generally at anypoint between where the imaging suspension is coated on the transparentelectrode and the imaging zone it is preferred that the film-splittingroller be located as close as possible to the area of contact betweenthe imaging roller and the transparent injecting electrode so as todecrease the time for dark discharge of the unipolar particles to occurprior to imaging.

Any suitable insulating carrier liquid may be used in the course of thepresent invention. Typical vehicles include decane, dodecane,tetradecane, molten paraffin wax, molten beeswax and other moltenthermoplastie materials. Sohio Odorless Solvent a kerosene fractionavailable from Standard Oil Company of Ohio, Isopar G a branched chainsaturated aliphatic hydrocarbon mixture available from Humble OilCompany of New Jersey, olive oil, mineral oil, linseed oil, cottonseedoil, marine oils such as sperm oil and cod liver oil, silicone coil suchas dimethyl polysiloxane (Dow Corning Co.), castor oil, corn oil, peanutoil, fluorinated hydrocarbons such as Freon (duPont) and compatiblemixtures thereof.

A wide range of voltages may be applied between the electrodes in thesystem at which imaging occurs. In the case of the field establishedacross the imaging suspension in the imaging zone it is preferred inorder to obtain good image resolution and density that the field acrossthe imaging suspension be at least volts/micron and preferably about 20volts/micron or more such as to create an electric field of at leastabout 300 volts. The applied potential necessary to obtain the field ofstrength will, of course, vary depending upon the interelectrode gap andupon the thickness and type of blocking material used on the respectiveimaging electrode surface. The preferred voltages normally exceed thecorona threshold at about 3500 volts in order to maintain the desiredlayering effect created by roller electrode 20 and to obviate prematuredark discharge and background migration of the particles. Voltages ashigh as 8000 volts have been applied to produce images of high quality.The upper limit of the field'strength is limited only by the breakdownpotential of the suspension and blocking material.

Imaging as carried out in conjunction with the process of the presentinvention will generally be in a negative to positive or positive tonegative imaging mode. Thus, for purposes of the present discussion, inorder to produce the positive image on the receiver sheet a negativeimage is projected into the nip of the imaging and transparentelectrodes. As discussed above a potential is applied aross the imagingsuspension. The pigment particles migrate upon exposure to the actinicradiation through the carrier to the surface of the imaging roller or,in the instance of the above described illustration, to the surface ofthe intervening receiver paper sheet. The pigment image formed may befixed in situ by placing a lamination over its surface or by solventremoval aided by the application of heat or if desired the image may betransferred to a secondary substrate to which it is in turn fixed. Thesystem herein described produces a high density image with little or nobackground.

Although represented as being formed on the surface of an interveningreceiver sheet, the pigment image may be formed on the surface of aremovable blocking layer or a transfer paper sleeve wrapped about theblocking electrode. In either instance the sleeve of paper material orthe blocking layer will pick up the complete image and need only beremoved to produce the final usable copy. All that is required is toreplace the removable material with a similar material. In the presentconfiguration images are produced directly on a paper receiver sheet orother substrate with the image formed on the NESA or transparentcylinder removed by the action of the ink applicator. However, ifdesired the image formed on the NESA cylinder need not be discarded butmay be utilized by offsetting the image from the NESA cylinder onto thesurface of a conventional receiving sheet such as described above. Ifthe image is formed on a permanent electrode surface it will be foundpreferable to transfer the image from the electrode and fix it on asecondary substrate so that the electrode may be reused. Such astransfer step may be carried out by' any suitable technique such asadhesive pick off techniques of preferably by electrostatic fieldtransfer. Any suitable material may be used as the receiving substratefor the image produced such as paper as represented in the illustrationor other desirable substrates. For example, if one desires to prepare atransparency the use of polyethylene terephthalate or cellulose acetatemight be desirable.

It is to be understood that it is not intended that the structuralarrangement of the apparatus represented by the illustration berestricted to the design as set out herein and all similarconfigurations which will satisfy the requirements of thepresentinvention are contemplated. For example, although the imagingelectrode or roller is represented as a cylinder it may also take theform of a flat plate electrode as may the injecting or NESA electrode.Furthermore, depending upon the specific configuration of the electrodesand other related aspects of the system either electrode whichparticipates in the direct imaging step could be optically transparentand exposure made through it.

When used in the course of the present invention the term injectingelectrode should be understood to mean that it is an electrode whichwill preferably be capable of exchanging charge with the photosensitiveparticles of the imaging suspension when the suspension is exposed tolight so as to allow for a net change in the charge polarity on theparticle. By the term blocking electrode or layer is meant one which issubstantially incapable of injecting charge carriers into thephotosensitive particles when the particles some into contact with thesurface of the respective electrode thereby eliminating particleoscillation in the system.

lt is preferred that the injecting electrode be composed of an opticallytransparent material, such as glass, overcoated with a conductivematerial such as tin oxide, copper, copper iodide, gold or the like;however other suitable materials including many semiconductive materialssuch as cellophane film, which are ordinarily not thought of as beingconductors but which are still capable of accepting injected chargecarriers of the proper polarity from the imaging particles under theinfluence of an applied electric field may be used within the course ofthe present invention. The use of more conductive materials allows forcleaner charge separation and prevents possible charge buildup on theelectrode. The blocking layer of the imaging electrode, on the otherhand, is selected so as to prevent or greatly retard the injection ofelectrons into the photosensitive pigment particles when the particlesreach the surface of this electrode. The core of the blocking or imagingelectrode generally will consist of a material which is fairly high inelectrical conductivity. Typical conductive materials includingconductive rubber, and metal foils of steel, aluminum, copper and. brasshave been found suitable. Preferably, the core of the electrode willhave a high electrical conductivity in order to establish the requiredfield differential in the system; however, if a material having a lowconductivity is used a separate electrical connection may be made to theback of the blocking layer of the blocking electrode. For example, theblocking layer or sleeve may be a semiconductive polyurethane materialhaving a conductivity of from about to 10 ohm-cm. If a hard rubbernon-conductive core is used then a metal foil may be used as a backingfor the blocking sleeve. Although a blocking layer need not necessarilybe used in the system, the use of such a layer is preferred because ofthe markedly improved results which it is capable of producing. It ispreferred that the blocking layer, when used, be either an insulatoror-a semi-conductor which will not allow for the passage of sufficientcharge carriers, under the influence of the applied field, to dischargethe particles finely bound to its surface thereby v preventing particleoscillation in the system. The result is enhanced image density andresolution. Even if the blocking layer does allow for the passage ofsome charge carriers to the photosensitive particles it still will beconsidered to fall within the class of preferred materials if it doesnot allow for the passage of sufficient charge so as to recharge theparticles to the opposite polarity. Exemplary of the preferred blockingmaterials used are baryta paper, Tedlar (a polyvinylfluoride), Mylar(polyethylene terephthalate) and polyurethane. Any other suitablematerial having a resistivity of from about 10 ohms-cm. or greater maybe employed. Typical materials in this resistivity range includecellulose acetate Coated papers, cellophane, polystyrene andpolytetrafluoroethylene. Other materials that may be used in theinjecting and blocking electrodes and other photosensitive particleswhich can be used as the photomigratory pigments and the variousconditions under which the system operates may be found in the abovecited issued patents US. Pat. Nos. 3,384,565 and 3,384,566 as well asUS. Pat. Nos. 3,384,488 and 3,383,993.

It is to be understood that any suitable photosensitive pigment particleas identified in the above cited patents may be employed within thecourse of the present invention with the selection depending largelyupon the photosensitivity and the spectral sensitivity required. Typicalphotoresponsive materials include substituted and unsubstituted organicpigments such as phthalocyanines, for example Monarch Blue G, beta formof copper phthalocyanine available from Hercules, lnc., quinacridones asfor example Monastral Red B available from duPont, Algol Yellow(l,2,5,6-di(C,C'- diphenyl)-diazoanthraquinone) (CI. 67300), lrgazineRed, tri-sodium salt of 2-carboxyl phenyl azo(2-naphthiol-3,6-disulfonic acid (CI. 16105), 3- benzylideneaminocarbazole, 3-aminocarbazole, Watchung Red B(l,4'-methyl-5'-chloroazobenzene- 2'-sulfonicacid)-2-hydroxy-3-naphthoic acid) (C.l. 15865), a yellow pigmentidentified as Yellow 96 comprisingN-2"pyridyl-8,13wdioxodinaphth0-(2,l-b; 2,3-d)-furan-6-carboxamide, andinorganic pigments such as zinc oxide, cadmium sulfide, cadmiumselenide, selenium, antimony sulfide, arsenic sulfide, and mixturesthereof. The imaging suspension may contain one or more differentphotosensitive particles having various ranges or spectral response.

PREFERRED EMBODIMENTS To further define the specifics of the presentinvention the following examples are intended to illustrate and notlimit the particulars of the present system. Parts and percentages areby weight unless otherwise indicated.

In the following examples the NESA electrode consists of a 6 inchdiameter Pyrex glass cylinder concentric to about 0.001 inch with aconductive tin oxide coating. The imaging or blocking electrode consistsof a 4 inch diameter conductive steel core with a /4 inch thick layer ofpolyurethane forming the blocking layer. Both the film-splitting(background control) and density control electrodes consist of a inchdiameter aluminum core covered with a A inch layer of polyurethane.

EXAMPLE I A cyan ink suspension consisting of 4.0 grams xphthalocyanine,2.0 grams tricresyl phosphate (TCP..0.05 grams beta carotene and aboutccs sperm oil is supplied to a tin oxide coated glass cylinder from aurethane sponge. The film'of imaging suspension is metered to athickness of about 3 microns as it passes beneath the density controlelectrode and a potential of about 1000 volts is applied. As the filmpasses the nip between the film-splitting roller and the NESA electrodea potential of about +7000 volts is applied across the imagingsuspension. As the imaging suspension proceeds to the nip between theNESA and imaging electrodes a negative image is projected into theimaging zone. A potential of about +8000 volts is developed across theimaging suspension during exposure. The speed of the imaging roller ismaintained at about 6 inches/second (ips). A 500 watt quartz iodinelight source illuminates the film negative. The light passes through anoptical system and is projected into the nip by way of a first surfacemirror. Cyan pigment particles are selectively deposited onto a paperreceiver sheet in the imaging zone. The x-phtalocyanine is preparedaccording to the process set out in US. Pat. No. 3,357,989, issued Dec.12, 1967, having a common assignee. High quality background free imagesare obtained with a background density of about 0.01 and a print densityof about 1.3 In a control experiment eliminating the density controlelectrode, at the above speed, the print density is about 0.5 withbackground about the same.

EXAMPLE 11 The process of Example 1 is repeated with the exception thatthe imaging suspension consists of a magenta ink suspension consistingof 8.0 grams Monastral Violet, 2.0 grams of TCP, 0.05 grams of betacarotene and 106 ccs sperm oil. The film is coated to a thickness ofabout 4 microns. The potential applied to the filmsplitting roller isabout +8000 volts and that applied to the density control roller isabout +1500 volts. Imaging speed is 12 inches/second. A magentaimage isformed on the surface of the paper receiver sheet which passes betweenthe nip of the NESA and imaging electrodes. The background densitymeasures 0.08 and print density 1.2. The Monastral Violet iscommercially availably from E.l. duPont de Nemours & Co. In a controlexperiment with no voltage applied to the density control electrodeprint density measured about 0.5 with the background remaining about thesame.

EXAMPLE Ill The process of Example 1 is repeatd with the exception thata yellow ink suspension comprising 20 grams Shepherd Golden Yellow No.55, 2 grams TCP, 0.05 grams beta carotene and 106 ccs Sohio brandkerosene is substituted for the cyan imaging suspension. The yellowpigment is commercially available from the Shepherd Chemical Company.Both the film-splitting and imaging rollers are operated at -6500 v.Imaging speed is about 45 inches/second. The density control roller isoperated at -l000 v. A high quality, low background yellow image isobtained having a background density of 0.02 and blue-light printdensity of 1.0. With the potential of the density control roller reducedto zero the density is observed to drop off to 0.6. Background is notappreciably affected.

EXAMPLE [V v The process of Example I is repeated with the exceptionthat a tri-mix imaging suspension is utilized in place of the cyansuspension. The tri-mix suspension consists of equal amounts of WatchungRed B, a barium salt of l-(4-methyl-5'-chloro-2-sulfonic acid)azobenzene-Z-hydroxy-3-naphthoic acid, C.l. No. 15865,

Monolite Fast Blue GS, a mixture of alpha and beta metal freephthalocyanine, available from Arnold Hoffman Co., C.l. No. 74100 and aproprietary yellow pigment N-2-pyridyl-8,13-dioxo-dinaphtho-(2l l-b;2',3- d) furan-6-carboxamide, more fully defined in US. Pat. applicationSer. No. 421,281 filed Dec. 28, 1964, now U.S. Pat. No. 3,447,922 havinga common assignee, in mineral oil with the total pigment consitutingabout 8 percent by weight of the imaging suspension. The inputinformation, is a Kodacolor negative. Imaging speed is 10 inches/second.A positive polychrone image is formed on the receiver sheet displaying alow background density of 0.03 and print density of 1.5. With theelimination of the density control roller print density drops to about0.8.

Although the present examples were specific in terms of conditions andmaterials used, any of the above materials may be substituted whensuitable with similar results being obtained. In addition to the stepsused to carry out the process of the present invention other steps ormodifications may be used if desirable. For example more than onedensity control electrode may be utilized. In addition, a polychromeimage may be formed by first preparing color separation negatives of acolor print and then utilizing the resulting color separation negativesto produce monochrome images of the corresponding colors in registrationat three separate imaging stations. Alternatively, each image may bereproduced and transferred in registration or each image may be producedon a single transparent sheet and the resulting imaged sheets placed oneon top of the other in registration to produce a transparent overlay forprojection purposes. In addition, other materials may be incorporated inthe imaging suspension, various different voltages may be applied, filmthicknesses utilized and the speeds may be varied in a manner which willenhance, synergize or otherwise desirably effect the properties of thepresent system. For example, various sensitizers may be included in theimaging suspension which will enhance the final results.

Those skilled in the art will have other modifications occur to thembased on the teachings of the present invention. These modifications areintended to be encompassed within the scope of this invention.

What is claimed is:

1. A photoelectrophoretic imaging apparatus comprising in combination:

a. an optically transparent first electrode adapted to support a layerof imaging suspension and adapted to have the surface of said electrodeadvanced relative to items (b), (c), (e), (f) and (h) recited below; b.means for introducing a layer of imaging suspension to the surface ofsaid optically transparent first electrode;

c. a second electrode mounted in close proximity to said opticallytransparent first electrode so as to make contact with a free surface ofthe layer of imaging suspension, at a station, in the direction ofadvancement of said optically transparent first electrode, after saidmeans for introducing a layer of imaging suspension;

(1. means to apply a first electrical potential difference across thelayer of imaging suspension between said optically transparent firstelectrode and said second electrode;

e. a third imaging electrode mounted in close proximity to saidoptically transparent first electrode so as to make contact with a freesurface of the layer of imaging suspension, at a station, in thedirection of advancement of said optically transparent first electrode,after said second electrode;

f. a fourth electrode mounted in close proximity to said opticallytransparent first electrode so as to make contact with a free surface ofthe layer of imaging suspension, at a station in the direction ofadvancement of said optically transparent first electrode after saidsecond electrode and before said third imaging electrode and mountedadjacent an imaging contact area established between said opticallytransparent first electrode and said third imaging electrode;

g. means to apply a second potential difference across the layer ofimaging suspension between said fourth electrode and said opticallytransparent first electrode;

h. means to expose with actinic electromagnetic radiation of an image tobe reproduced, that portion of the layer of imaging suspension at anarea of contact of said third imaging electrode and said opticallytransparent first electrode; and

. means to apply a third potential difference across the layer ofimagingsuspension, between said third imaging electrode and said opticallytransparent first electrode during said exposure.

2. Apparatus according to claim 1 wherein said means to apply a thirdpotential difference comprises means to apply a potential difference atleast about 3,500 volts D.C.

3. Apparatus according to claim 1 further comprising means for advancinga receiving member between said optically transparent first electrodeand said third imaging electrode at the area of contact of said thirdimaging electrode and said optically transparent first electrode.

4. Apparatus according to claim 1 wherein said means to expose isadapted to project an image along the longitudinal axis of saidtransparent electrode into the nip of said optically transparent firstelectrode and said third imaging electrode.

5. Apparatus according to claim 1 wherein said means to apply a firstpotential difference comprises means to apply a potential differencebelow the corona threshold for the air gap between the layer of imagingsuspension and said second electrode.

6. Apparatus according to claim 5 wherein said means to apply a firstpotential difference comprises means to apply a potential differencebelow about 13 2,500 volts and of the same polarity as said secondpotential difference.

7. Apparatus according to claim 1 wherein said means to apply a secondpotential difference comprises means to apply a potential differenceabove the corona threshold for the air gap between the layer of imagingsuspension and said fourth electrode.

8. Apparatus according to claim 7 wherein said means to apply a secondpotential difference comprises means to apply a potential differenceabove about 3,500 volts DC. and of the same polarity as said firstpotential difference.

9. Apparatus according to claim 1 wherein each of said electrodes is arotatably mounted roller.

10. Apparatus according to claim 9 further comprising means for rotatingeach of said electrodes in synchronization.

11. Apparatus according to claim 1 further comprising means forcontrolling said first potential difference so as to maintainconcentration of pigment in the layer of imaging suspension relativelyconstant and at the desired level.

12. Apparatus according to claim 11 wherein said means for controllingcomprises a photocell to measure density of the layer of imagingsuspension between said second and said fourth electrodes.

13. A photoelectrophoretic imaging apparatus comprising in combination:I

a. an optically transparent, flat plate, first electrode adapted tosupport a layer of imaging suspension;

b. means for introducing a layer of imaging suspension to the surface ofsaid optically transparent, flat plate, first electrode;

0. a second electrode mounted in close proximity to said first electrodesuch that said second electrode can make progressively advancing contactwith a free surface of a layer of imaging suspension on the surface ofsaid optically transparent, flat plate, first electrode;

d. means to apply a first potential difference across the layer ofimaging suspension, between said optically transparent, flat plate,first electrode and said second electrode; 7 e. a third, imagingelectrode mounted in close proximity to said optically transparent, flatplate, first electrode at a station in the direction of advancement ofsaid second electrode, after said second electrode such that said third,imaging electrode can make progressively advancing contact with a freesurface of a layer of imaging suspension on the surface of saidoptically transparent, flat plate, first electrode;

f. a fourth electrode mounted in close proximity to said opticallytransparent, flat plate, first electrode so as to make surface contactwith a free surface of the layer of imaging suspension and at a stationbetween said second elecrode and said third, imaging electrode;

g. means to apply a second potential difference across the layer ofimaging suspension between said fourth electrode and said opticallytransparent, flat plate, first electrode;

h. means to expose with actinic electromagnetic radiation of an image tobe reproduced, that portion of the layer of imaging suspension at anarea of contact of said third, imaging electrode and said opticallytransparent, flat plate, first electrode;

means to apply a third potential difference across the layer of imagingsuspension between said third, imaging electrode and said opticallytransparent, first electrode during said exposure; and j; means to causerelative movement between said optically transparent, flat plate, firstelectrode and said second, third and fourth electrodes.

14. Apparatus according to claim 13 futher comprising means to disengagesaid optically transparent, flat plate, first electrode from saidremaining electrodes.

15. Apparatus according to claim 13 wherein said second electrode, saidthird, imaging electrode and said fourth electrode are rotatably mountedrollers.

, UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.3,784,294 Dated January 8, 1974 Inventor(s) John B. Wells It iscertified that error appears in the above-identified patent and thatsaid Letters Patent are hereby corrected as shown below:

Column 4, line 67, delete "photo-conductive" and insert 4--photoconductive--.

Column 6, line 42, delete "filed" and insert -field--.

Column 6, lines 52-53, delete "acceleraed" and insert -acce lerated--,

Column 7, line 7, delete "coil" and insert oi l.

Column 7, line 39, delete "aross" and insert -across--.

Column 8, line 4, delete as and insert --a.

Column 8 line 35, delete "some" and insert come-,.

Column 10, line 37, delete "1.3 In" and insert -l.3. In.

Column 10, line 51, delete "magentaimage" and insert -magenta image.

Column 10, line 63, delete "repeatd" and insert -repeated.

Signed and sealed this 23rd day of July 1974.

(SEAL) Attest:

MCCOY M. GIBSON, JR. 0. MARSHALL DANN Attesting Officer Commissioner ofPatents UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.3'784I294 Dated January 1974 Invent0r(s) John B. Wells It is certifiedthat error appears in the above-identified patent and that said LettersPatent are hereby corrected as shown below:

I I I Column 4, line 67, delete "photo-conductive" and insertphotoconductive-.

Column 6, line 42 delete "filed" and insert --fie1d-.

Column 6, lines 52-53, delete "acceleraed" and insert acce 1erated--.

Column 7, line 7, delete "coil" and insert --oi1.

Column 7, line 39, delete "aross" and insert -across.

Column 8, line 4, delete "as" and insert -a.

Column 8 line 35, delete "some" and insert -come:-.

Column 10, line 37, delete "1.3 In" and insert --1.3. In--.

Column 10, line 51, delete "magentaimage" and insert -magenta image.

Column 10, line 63, delete "repeatd" and insert repeated-.

Signed and sealed this 23rd day of July 1971 (SEAL) Attest:

MCCOY M. GIBSON, JR. C. MARSHALL DANN Attesting Officer Commissioner ofPatents

1. A photoelectrophoretic imaging apparatus comprising in combination:a. an optically transparent first electrode adapted to support a layerof imaging suspension and adapted to have the surface of said electrodeadvanced relative to items (b), (c), (e), (f) and (h) recited below; b.means for Introducing a layer of imaging suspension to the surface ofsaid optically transparent first electrode; c. a second electrodemounted in close proximity to said optically transparent first electrodeso as to make contact with a free surface of the layer of imagingsuspension, at a station, in the direction of advancement of saidoptically transparent first electrode, after said means for introducinga layer of imaging suspension; d. means to apply a first electricalpotential difference across the layer of imaging suspension between saidoptically transparent first electrode and said second electrode; e. athird imaging electrode mounted in close proximity to said opticallytransparent first electrode so as to make contact with a free surface ofthe layer of imaging suspension, at a station, in the direction ofadvancement of said optically transparent first electrode, after saidsecond electrode; f. a fourth electrode mounted in close proximity tosaid optically transparent first electrode so as to make contact with afree surface of the layer of imaging suspension, at a station in thedirection of advancement of said optically transparent first electrodeafter said second electrode and before said third imaging electrode andmounted adjacent an imaging contact area established between saidoptically transparent first electrode and said third imaging electrode;g. means to apply a second potential difference across the layer ofimaging suspension between said fourth electrode and said opticallytransparent first electrode; h. means to expose with actinicelectromagnetic radiation of an image to be reproduced, that portion ofthe layer of imaging suspension at an area of contact of said thirdimaging electrode and said optically transparent first electrode; and i.means to apply a third potential difference across the layer of imagingsuspension, between said third imaging electrode and said opticallytransparent first electrode during said exposure.
 2. Apparatus accordingto claim 1 wherein said means to apply a third potential differencecomprises means to apply a potential difference at least about 3,500volts D.C.
 3. Apparatus according to claim 1 further comprising meansfor advancing a receiving member between said optically transparentfirst electrode and said third imaging electrode at the area of contactof said third imaging electrode and said optically transparent firstelectrode.
 4. Apparatus according to claim 1 wherein said means toexpose is adapted to project an image along the longitudinal axis ofsaid transparent electrode into the nip of said optically transparentfirst electrode and said third imaging electrode.
 5. Apparatus accordingto claim 1 wherein said means to apply a first potential differencecomprises means to apply a potential difference below the coronathreshold for the air gap between the layer of imaging suspension andsaid second electrode.
 6. Apparatus according to claim 5 wherein saidmeans to apply a first potential difference comprises means to apply apotential difference below about 2,500 volts and of the same polarity assaid second potential difference.
 7. Apparatus according to claim 1wherein said means to apply a second potential difference comprisesmeans to apply a potential difference above the corona threshold for theair gap between the layer of imaging suspension and said fourthelectrode.
 8. Apparatus according to claim 7 wherein said means to applya second potential difference comprises means to apply a potentialdifference above about 3,500 volts D.C. and of the same polarity as saidfirst potential difference.
 9. Apparatus according to claim 1 whereineach of said electrodes is a rotatably mounted roller.
 10. Apparatusaccording to claim 9 further comprising means for rotating each of saidelectrodes in synchronization.
 11. Apparatus according to claim 1further comprising means for controlling said first potential differenceso as to maintAin concentration of pigment in the layer of imagingsuspension relatively constant and at the desired level.
 12. Apparatusaccording to claim 11 wherein said means for controlling comprises aphotocell to measure density of the layer of imaging suspension betweensaid second and said fourth electrodes.
 13. A photoelectrophoreticimaging apparatus comprising in combination: a. an opticallytransparent, flat plate, first electrode adapted to support a layer ofimaging suspension; b. means for introducing a layer of imagingsuspension to the surface of said optically transparent, flat plate,first electrode; c. a second electrode mounted in close proximity tosaid first electrode such that said second electrode can makeprogressively advancing contact with a free surface of a layer ofimaging suspension on the surface of said optically transparent, flatplate, first electrode; d. means to apply a first potential differenceacross the layer of imaging suspension, between said opticallytransparent, flat plate, first electrode and said second electrode; e. athird, imaging electrode mounted in close proximity to said opticallytransparent, flat plate, first electrode at a station in the directionof advancement of said second electrode, after said second electrodesuch that said third, imaging electrode can make progressively advancingcontact with a free surface of a layer of imaging suspension on thesurface of said optically transparent, flat plate, first electrode; f. afourth electrode mounted in close proximity to said opticallytransparent, flat plate, first electrode so as to make surface contactwith a free surface of the layer of imaging suspension and at a stationbetween said second elecrode and said third, imaging electrode; g. meansto apply a second potential difference across the layer of imagingsuspension between said fourth electrode and said optically transparent,flat plate, first electrode; h. means to expose with actinicelectromagnetic radiation of an image to be reproduced, that portion ofthe layer of imaging suspension at an area of contact of said third,imaging electrode and said optically transparent, flat plate, firstelectrode; i. means to apply a third potential difference across thelayer of imaging suspension between said third, imaging electrode andsaid optically transparent, first electrode during said exposure; and j.means to cause relative movement between said optically transparent,flat plate, first electrode and said second, third and fourthelectrodes.
 14. Apparatus according to claim 13 futher comprising meansto disengage said optically transparent, flat plate, first electrodefrom said remaining electrodes.
 15. Apparatus according to claim 13wherein said second electrode, said third, imaging electrode and saidfourth electrode are rotatably mounted rollers.